The present invention relates generally to the field of thermoelectric devices and more particularly to the thermoelectric cooling system with improved performance and/or efficiency characteristics. The present invention is designed to have many applications including use in a bread box, as part of a wine rack, or in any other application requiring the cooling of a load.
Thermoelectric cooling systems are analogous to conventional refrigeration cooling systems. For example, a conventional cooling system includes an evaporator, a compressor, and a condenser. In the evaporator or cold section, pressurized refrigerant is allowed to expand, boil, and evaporate. During the change of state from a liquid to a gas, energy in the form of heat is absorbed. In the next step, the compressor re-compresses the gas into a liquid. Further, the condenser expels the heat absorbed at the evaporate and the extra heat added by the compressor to the ambient environment.
A thermoelectric cooling system has similar subassemblies. However, thermoelectric cooling is specifically the abstraction of heat from electronic components by Peltier effect, greatly improved and made practicable with solid-state thermoelectric materials, e.g., Bi2Te3. Devices using this effect, e.g. frigistors, are used for automatic temperature control, and the like and are energized by direct current (xe2x80x9cd.c.xe2x80x9d) thermoelectric materials, that is, any set of materials (metals) which constitute a thermoelectric system. Some examples include: xe2x80x9cbinaryxe2x80x9d systems (bismuth and tellurium), xe2x80x9cternaryxe2x80x9d systems (silver, antimony and tellurium), and xe2x80x9cquaternaryxe2x80x9d systems (bismuth, tellurium, selenium and antimony, called xe2x80x9cNeeliumxe2x80x9d). The Peltier effect is a phenomenon whereby heat is liberated or absorbed at a junction when current passes from one metal to another. In this application, a cold junction (the place where the heat source or load is located) is defined as the assembly where energy in the form of heat is absorbed when current passes from one metal to another. A hot junction (the place where the heat sink is located) is the assembly which thermally communicates with a heat exchanger and through which the heat that is liberated, when current passes from one metal to another, is transferred to the ambient environment.
Major differences exist between thermoelectric cooling systems and conventional refrigeration systems, however. For example, conventional refrigeration systems must maintain a closed environment isolated from the ambient. Further, conventional refrigeration systems have a large amount of insulation and cannot be ventilated without loss of cooling effect. Thus, conventional cooling systems may contain odors of the loads placed within and such odors may be transferred to other loads placed within the cooling system, with obviously undesirable results. Further, conventional cooling systems may adversely affect the physical characteristics of the product being cooled, such as texture, taste, shelf life, and the like, of certain food articles which may be placed therein. For example, fresh baked bread may, if humidity and temperature are not carefully controlled, become soggy on at least one side during the cooling process.
Thermoelectric cooling systems, by contrast, provide a measure of advantage to the several shortcomings noted above. However, thermoelectric cooling systems of the prior art lack efficiency in certain respects because, upon interruption of the power supply, the current reverses flow such that what was a heat source becomes the heat sink, and what was the heat sink now becomes the heat source. Additionally, thermoelectric cooling systems of the prior art tend to suffer from moisture condensation problems near the thermoelectric chip or the cooling plate.
The invention may be generally described as a thermoelectric cooling system having an electric circuit comprising a direct current (xe2x80x9cd.c.xe2x80x9d) power source for providing direct current throughout the electric circuit, a thermoelectric module having at least one heat sink and at least one heat source capable of being cooled to a predetermined temperature range, and a control assembly. The d.c. power source, the control assembly, and the thermoelectric module are connected to each other in series. In one optional embodiment, the control assembly comprises a thermostat control switch mechanism and a resistive element connected to each other in parallel. In such an embodiment, the thermostat control switch mechanism has a sensor coupled to or thermally associated with the heat source of the thermoelectric module so that the temperature of the heat source can be monitored. The thermostat control switch mechanism is normally open in the predetermined temperature range detected by the sensor. The resistive element having a predetermined resistance sufficient for a level of voltage to be provided to the thermoelectric module, when the thermostat control switch mechanism is open, sufficient to substantially prevent reversal of the heat source and the heat sink.
In an alternate embodiment, the control assembly comprises a relay with a voltage generating thermocouple. In this embodiment, the relay senses the voltage from the thermocouple. Upon sensing a predetermined threshold level, the relay is energized and provides a level of voltage to the interface sufficient to substantially prevent reversal of the heat source and the heat sink.
More particularly, the circuit of the present invention may be used in a thermoelectric bread box or a thermoelectric wine bottle cooling rack. The thermoelectric bread box maintains freshness by keeping wrapped bread at a specified temperature range to prevent spoilage by mold growth. The device is almost devoid of insulation except to prevent condensation inside the thermoelectric element. The conforming cooling plate that is placed around the load is a substantially U-shaped absorber to remove the heat. From test data it appears that when a conforming cooling plate is used objects cool faster. When conformal cooling plates or heat absorbers are employed on a bottle or container of liquid, e.g., wine, stratification is prevented and circulation is promoted inside the bottle which aids cooling. An explanation for this effect on liquids or wrapped bread may be that long wave radiation frequencies of 2 microns or greater, coming from wrapped bread, glass bottles, or other containers are absorbed by a conformal aluminum or copper plate (or plate having similar characteristics) if it surrounds a substantial portion, at least 75%, of the object to be cooled. An alternative explanation may be that substantially all the surface area of the body being cooled is associated with the conformal cooling surface. This method cools a bottle of water faster than a refrigerator using circulated cold air. This can be accomplished without insulating the entire box.
Alternatively, it has recently been found that effective cooling of liquids, like wine, in containers, like bottles, may be accomplished with a conformal cooling plate (first body) in cooling association with less than 75% of the object to be cooled, provided, a portion of the conformal cooling plate is in cooling contact or association with at least a portion of one side of the container or object to be cooled (the load). Liquids cooled by such a structure, or such a process, avoid stratification and substantially unequal cooling. Accordingly, uniform cooling of a liquid and maintenance of such uniform cooling may be accomplished by use of this alternative structure and method.
The thermoelectric cooling system of the present invention may likewise be described as a first body which is the heat source, a second body which is the heat sink, and an interface composed of thermoelectric materials thermally connecting or coupling the first body with the second body. The interface is connected in a series to a d.c. power source. The first body is operable to absorb heat and thereafter transfer the heat to the second body through the interface, by the Peltier effect, when current is applied to the interface from the d.c. power source. The thermoelectric cooling system further includes a control assembly. As discussed above, in a first embodiment, the control assembly is a thermostat and a resistor which are electrically connected in parallel to one another. The parallel connection of the thermostat and the resistor creates a control device which allows the flow of current in a direction which prevents movement of heat from the second body to the first body when the thermostat control switch is opened. A minimum voltage is retained in the circuit and the heat return to the first body is thereby minimized. In a second alternate embodiment, the control assembly is a relay and a thermocouple. The thermocouple applies a voltage to trip the relay which raises the level of voltage to the interface sufficient to substantially prevent reversal of the heat source and the heat sink.
The first body includes a shelf or conforming plate mounted within a housing, and wherein the load or article of interest (typically articles of food or beverage containers) to be cooled is placed on the shelf or within the conforming plate. The shelf or conforming plate are preferably made of a highly heat conductive material like metal and may have a uniform thickness or optionally, varying thickness. Alternatively, the load may rest on an isolated surface below a shelf. The load is then cooled by convection rather than conduction. The second body, made of a heat conductive material, includes a radiator, preferably, having a plurality of fins which radiate heat therefrom. The design of the first body and the second body is such that a steady state device is created, that is, the radiator emanates an amount of heat which is substantially equivalent to the heat generated by the mass of the shelf and the load on the shelf and the heat produced by the electrical circuit. The invention may further include a fan directing heated air from the second body to an enclosed area to create an area of higher temperature.
The interface comprises a semiconductor or plurality of semiconductors which produce the desired Peltier effect. Varying numbers and types of semiconductors or other suitable thermoelectric materials may be employed to achieve different degrees of cooling.
The housing may include several apertures which serve to ventilate the load placed upon the shelf and cooled by the effects of conduction. The present invention requires little, if any, insulation. The housing may also include an external rack.
As a means of moisture control, various elements may be placed in thermal association with the first body, the interface, or both. In one embodiment of the present invention, a metal strap is disposed intermediate the interface and the first body such that the first body, metal strap, and interface are mechanically connected in series. The area over which the strap and first body contact is much smaller than the total surface area of the first body thereby restricting the amount of heat transferred from the first body to the interface. In this fashion, the first body may be maintained above the dew point of the surrounding air.
In an alternate embodiment, a moisture control pad is disposed in thermal association with the first body, the interface, or both. For example, in an optional embodiment, a moisture control pad is disposed intermediate the interface and the first body. Similarly, in an optional embodiment, the moisture control pad may be disposed on the surface of the first body facing toward the load. The moisture control pad is formed from a material that absorbs moisture and permits re-evaporation of the absorbed moisture to substantially maintain a desired humidity range. Suitable materials for the moisture control pad include pulp fiber paper.